The Lancefield Group H has been shown to define the species Streptococcus sanguis. The group antigen appears to be a lipoteichoic acid which is extracted by the Rantz and Randall autoclave technique. At the present time this procedure appears to be a better method for serological classification of this group than other extraction procedures which give greater yields of the antigen but which result in cross reactions with serological groups D and N; these antigens are also lipoteichoic acids. Immunochemical studies of the isolated antigens are underway which will seek to determine the basis for these findings. Using this taxonomic scheme as a starting point, a number of properties, i.e., cell wall composition, salivary aggregation, lysogenic conversion, transformation and production of autolytic enzymes, will be examined to determine if a relationship exists between these properties and the antigenic structure of these cells. Because most of all of these properties involve polymers which are antigenic, an immunochemical approach to the study has been adopted. It is hoped that these studies will enable us to determine if modulation (changes in the antigenic structure) of dental plaque is regulated by continuous reinfection with new serotypes or whether an internal genetic mechanism among indigenous flora is responsible for modulation. If specific serotypes are involved in plaque formation, then it might be possible to prepare specific immunological or chemical reagents which could inhibit activities or "block" polymers responsible for plaque development. BIBLIOGRAPHIC REFERENCES: Lai, C., Listgarten, M., Rosan, B. Immunoelectron microscopic identification and localization of Streptococcus sanguis with peroxidase-labelled antibody: localization of Streptococcus sanguis in intact dental plaque. Infect. Immun. 11: 200-210, 1975. Rosan, B., Lai, C., Listgarten, M. Streptococcus sanguis: A model in the application of immunochemical for the in situ localization of bacteria in dental plaque. J. Dent. Res., 55: A124-A141, 1976.